Redox-Mediated Reconstruction of Copper during Carbon Monoxide Oxidation

Copper has excellent initial activity for the oxidation of CO, yet it rapidly deactivates under reaction conditions. In an effort to obtain a full picture of the dynamic morphological and chemical changes occurring on the surface of catalysts under CO oxidation conditions, a complementary set of in situ ambient pressure (AP) techniques that include scanning tunneling microscopy, infrared reflection absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy were conducted. Herein, we report in situ AP CO oxidation experiments over Cu(111) model catalysts at room temperature. Depending on the CO:O₂ ratio, Cu presents different oxidation states, leading to the coexistence of several phases. During CO oxidation, a redox cycle is observed on the substrate’s surface, in which Cu atoms are oxidized and pulled from terraces and step edges and then are reduced and rejoin nearby step edges. IRRAS results confirm the presence of under-coordinated Cu atoms during the reaction. By using control experiments to isolate individual phases, it is shown that the rate for CO oxidation decreases systematically as metallic copper is fully oxidized

<i>In Situ</i> Imaging of Cu₂O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of <i>in situ</i> microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu₂O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution <i>in situ</i> imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces

<i>In Situ</i> Imaging of Cu₂O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of <i>in situ</i> microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu₂O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution <i>in situ</i> imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces

<i>In Situ</i> Imaging of Cu₂O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of <i>in situ</i> microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu₂O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution <i>in situ</i> imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces

Unraveling the Dynamic Nature of a CuO/CeO₂ Catalyst for CO Oxidation in <i>Operando</i>: A Combined Study of XANES (Fluorescence) and DRIFTS

The redox chemistry and CO oxidation (2CO + O₂ → 2CO₂) activity of catalysts generated by the dispersion of CuO on CeO₂ nanorods were investigated using a multitechnique approach. Combined measurements of time-resolved X-ray absorption near-edge spectroscopy (XANES) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in one setup were made possible with the development of a novel reaction cell in which fluorescence mode detection was applied to collect the XANES spectra. This is the first reported example using XANES in a similar technique combination. With the assistance of parallel time-resolved X-ray diffraction (XRD) measurements under <i>operando</i> conditions, we successfully probed the redox behavior of CuO/CeO₂ under CO reduction, constant-flow (steady-state) CO oxidation and during CO/O₂ cycling reactions. A strong copper ↔ ceria synergistic effect was observed in the CuO/CeO₂ catalyst. Surface Cu­(I) species were found to exhibit a strong correlation with the catalyst activity for the CO oxidation reaction. By analysis of phase transformations as well as changes in oxidation state during the nonsteady states in the CO/O₂ cycling reaction, we collected information on the relative transformation rates of key species. Elementary steps in the mechanism for the CO oxidation reaction are proposed based on the understandings gained from the XANES/DRIFTS <i>operando</i> studies